Multiplex systems having duplex communication arrangements are well known in the art. Essentially all modern, complex electronically controlled systems include arrangements for providing communication among various functions or parts of the system as, for example, among a master control key telephone station and a number of associated station sets. By means of such intercommunication, the control station, acting on information received from the station sets and perhaps from elsewhere, typically provides information or instructions back to the station sets.
Asynchronous multiplex systems, such as those disclosed in U.S. Pat. No. 4,097,695 issued to A. G. V. Grace et al. on June 27, 1978, and U.S. Pat. No. 4,325,147 issued to A. P. Rothlauf on Apr. 13, 1982, are known in the art. In such systems, the bits within each character or block of characters relate to a fixed time frame, but wherein the start of each character or block thereof is not necessarily related to such fixed time frame.
Asynchronous multiplex systems avoid the need for transmitting high frequency clock signals between stations by employing at each station an integrated circuit known as a universal asynchronous receiver transmitter (UART). These UARTs have simultaneous receiving and transmitting capabilities and are themselves complete subsystems. They provide the required formatting for converting data words serially received from an external station into parallel format for use by the associated station, and for serially transmitting to an external station those data words input in parallel format by the associated station. Although timing clocks are necessary, the clocks employed by each UART in a pair of stations between which information is communicated need not be synchronous; thus allowing for independent clock generators to be used.
Software UARTs are known to exist in the art. These UARTs, however, are capable of providing only low rate asynchronous communications between stations. Communications with such a UART is achieved by configuring a computer so that it sends and receives serial data through its regular input and output ports and performs the serial to parallel conversion of the data in software. This type of software UART consumes considerable processor time, however, and is only suitable for communicating at relative slow baud rates, i.e., 300 to 1200 baud.
High speed asynchronous multiplex systems in the known art all use standard integrated circuit UARTs at both the sending and receiving station in order to provide the exchange of information at the required rate. These UARTs are complex devices and require considerable circuit integration for the desired operation. With the low cost of many integrated circuits today, including integrated single chip microcomputers, UARTs are comparatively expensive devices. Thus in those system designs having multiple stations and where low cost and high speed are considerations, a requirement of using a UART at each station can easily make the design unattractive. It is desirable, therefore, to have a multiplexed system which allows for asynchronous exchange of information at the desired high rate without the requirement of having a UART associated with each station.